With WET you have a real flat bridge (wireless ethernet transport).
The WRT simply acts as a dumb layer2-bridge and the clients behind it's LAN-port have to be in the same subnet as the main-router (if you use DHCP, they get their adress from the main-router).
If you use client-mode then you have your own subnet behind the router (like you were connected to the internet) and the only device which is associated with your main-router is the WRT.